In Situ Remediation of a TCE-Contaminated Aquifer Using a Short Rotation Woody Crop Groundwater Treatment System Naval Air Station Joint Reserve Base, Fort Worth, Texas

Site Name:

Naval Air Station Joint Reserve Base (NAS-JRB)

Location:

Fort Worth, Texas

Period ofOperation:

August 1996 to September 1998

CleanupType:

Field Demonstration

Technology:Phytoremediation:
-- The primary objective of the demonstration was to study the mechanism of phytocontainment. Phytocontainment is achieved via transpiration (the evaporative loss of water from a plant). Eastern cottonwood trees were chosen as the preferred vegetation for this demonstration. They are classified as a short rotation woody crop (SRWC) because they are fast-growing and are easy to regenerate.
-- The SRWC groundwater treatment (SRWCGT) system consisted of two 15 x 75 square meter (m2) plantations, one planted with seven rows of whips or 1-year old stem cuttings (438 total) and the other planted with seven rows of caliper trees or 1-year old seedlings (224 total). A total of 662 trees were planted at the site. The two sizes of trees were selected for planting so that differences in rate of growth, contaminant reductions, and cost based on planting strategy could be compared.
-- Both plantations were oriented generally perpendicular to groundwater flow direction and spanned the most concentrated portion of the underlying TCE-groundwater plume.
-- Contrary to many conventional treatment processes, a SRWCGT system does not require the addition of any chemical or biological enhancements.

Waste Source:Historically, manufacturing processes at Plant 4 of the NAS-JRB generated an estimated 5,500 to 6,000 tons of waste per year, including: waste solvents, oils, fuels, paint residues, and miscellaneous spent chemicals. TCE is believed to have leaked from degreasing tanks in the assembly building at Plant 4 and entered the underlying alluvial aquifer.

Type/Quantity of Media Treated:Groundwater (quantity not specified)

Purpose/Significance of Application:To evaluate the capability of Eastern cottonwood trees (Populus deltoides) to intercept and treat groundwater contaminated with TCE and c-DCE.

The primary objective of the SRWCGT system focused on localized hydraulic containment and the goals were to:
-- Achieve a 30% reduction in the mass of TCE in the aquifer that is transported across the downgradient end of the site during the second growing season, relative to baseline TCE mass flux calculations.
-- Achieve a 50% reduction in mass of TCE in the aquifer that is transported across the downgradient end of the site during the third growing season, relative to baseline TCE mass flux calculations.

Results:The SRWCGT system did not achieve the mass flux reductions goal of 30% and 50% for the second and third growing seasons, respectively. For the second growing season, the TCE mass flux was up 8% during peak season, as compared to baseline conditions. The planted trees reduced the outward flux of groundwater by 5% during the peak of the second season, but TCE concentrations in a row of wells immediately downgradient of the trees were higher, resulting in the increase in TCE mass flux. For the third growing season, the TCE mass flux was down 11% at peak season and down 8% near seasons end, as compared to baseline conditions. Concentrations of TCE during the third season in the row of downgradient wells were similar to concentrations at baseline, and the reduction in TCE mass flux is primarily attributed to a reduction in the volumetric flux of groundwater out of the site. The primary objective was not met because the trees did not reach their full transpiration potential during the time period of the demonstration study, but greater hydraulic control at the site is anticipated in the future.

The data show a general decrease in TCE concentrations throughout the demonstration site over the course of the study. However, since a decrease in TCE concentration was observed in the upgradient monitoring wells as well as in the wells within the plantations, this trend does not appear to be predominantly related to the establishment of the whip and caliper tree plantations. Secondly, downgradient monitoring wells did not exhibit a
significant decrease in TCE concentrations. The change in TCE concentrations within the study area over time may be attributed to dilution from recharge to the aquifer and volatilization of TCE from the water table.

Cost Factors:Total estimated demonstration costs were $641,467, which included $426,427 in actual labor costs, $172,740 in other direct costs and $42,300 in laboratory costs.

Description:The site chosen for the demonstration was a DoD site with a large unattenuated contaminant plume due to the lack of adequate amounts of native and/or anthropogenic carbon and dissolved oxygen (DO) levels. The site was selected to demonstrate the SRWCGT system because of its geographical location, type of contamination, and depth of contamination. The site specifically exhibited the following characteristics:
-- Type-3 conditions (i.e., DO levels >1 mg/L and a lack of carbon sources that prevented reductive dechlorination of chlorinated compounds).
-- The groundwater at the site is shallow and thus accessible to trees soon after planting.
-- An ample area, clear of obstructions, was available for plantations (i.e., the technology is well suited for use at very large field sites where other methods of remediation are not cost effective or practical).
-- The site allowed for long-term, field-scale monitoring and evaluation.
-- Previously installed wells were available to monitor the treatment system (water levels in wells provide a direct means for assessing groundwater uptake by the trees).

The site selected for the demonstration was an approximate 70-m-wide portion of a TCE plume on the north side of the site. Specifically, the study was undertaken to determine the potential for a SRWC to decrease TCE flux. Although TCE was the focus of the demonstration, other chlorinated organic compounds detected in the groundwater or plant tissue included, but were not limited to, cDCE, tDCE, PCE, methylene chloride, toluene, and VC.